How much water should I drink a day?

A common starting point is about 35 ml of water per kilogram of bodyweight per day — roughly 2.5 litres for a 70 kg person. Add about 350 ml for every 30 minutes of exercise. Treat this as a baseline guide and adjust for heat, sweat, and how thirsty you feel.

Does the "8 glasses a day" rule hold up?

The "8 × 8" rule (eight 8-ounce glasses, about 1.9 litres) is a memorable rule of thumb, not a strict requirement. It is in the right ballpark for many people but ignores bodyweight, activity, and climate. Scaling to your weight, as this calculator does, is usually closer.

How much extra water should I drink for exercise?

This calculator adds roughly 350 ml per 30 minutes of exercise to cover sweat losses. Long or intense sessions in the heat can lose far more, so drink to thirst during training and rehydrate afterwards based on how much weight you sweated off.

Do coffee and tea count toward water intake?

Yes. Despite their mild diuretic effect, the fluid in coffee, tea, and most other drinks still contributes to your daily total, and water-rich foods like fruit and vegetables add more. Plain water is simplest to track, but it is not the only source.

Can you drink too much water?

It is possible but uncommon. Drinking very large volumes in a short time can dilute blood sodium (hyponatraemia), which is dangerous. Spreading your intake through the day and drinking to thirst rather than forcing litres at once keeps you safe.

Water Intake Calculator

This water intake calculator estimates how much water to drink per day from your bodyweight and daily exercise time. It uses a simple bodyweight baseline and gives the result in litres, ounces and 8-oz cups.

How it works

The calculator starts from a widely used rule of thumb: 35 ml of water per kilogram of bodyweight per day. It then adds 350 ml for every 30 minutes of exercise to cover the extra fluid you lose through sweat. In short:

water (ml) = bodyweight (kg) × 35 + (exercise minutes ÷ 30) × 350

Litres, ounces and cups are just conversions of that millilitre total (1 US fluid ounce = 29.5735 ml; one cup here is 8 fl oz).

Worked example

An 80 kg person who trains for 60 minutes: the baseline is 80 × 35 = 2800 ml (2.8 L), and exercise adds (60 ÷ 30) × 350 = 700 ml (0.7 L). The total is 3.5 L per day — about 118 oz or 15 cups.

Baseline water by bodyweight

Baseline daily water (before any exercise), at 35 ml per kg:

Bodyweight	Baseline (L/day)	Baseline (oz/day)
50 kg (≈110 lb)	1.75	59
70 kg (≈154 lb)	2.45	83
90 kg (≈198 lb)	3.15	107

Add about 350 ml (≈12 oz) for every 30 minutes of exercise on top of these baselines.

How to read the result

The figure is a starting target, not a hard rule. Hot weather, high altitude, illness, breastfeeding and pregnancy all raise your needs, while a lot of your intake also comes from food and other drinks. Use the number as a daily anchor and let thirst and urine colour — pale straw is a good sign — fine-tune it. To pair hydration with energy needs, see the TDEE calculator.

Frequently asked questions

How much water should I drink a day?: A common starting point is about 35 ml of water per kilogram of bodyweight per day — roughly 2.5 litres for a 70 kg person. Add about 350 ml for every 30 minutes of exercise. Treat this as a baseline guide and adjust for heat, sweat, and how thirsty you feel.
Does the "8 glasses a day" rule hold up?: The "8 × 8" rule (eight 8-ounce glasses, about 1.9 litres) is a memorable rule of thumb, not a strict requirement. It is in the right ballpark for many people but ignores bodyweight, activity, and climate. Scaling to your weight, as this calculator does, is usually closer.
How much extra water should I drink for exercise?: This calculator adds roughly 350 ml per 30 minutes of exercise to cover sweat losses. Long or intense sessions in the heat can lose far more, so drink to thirst during training and rehydrate afterwards based on how much weight you sweated off.
Do coffee and tea count toward water intake?: Yes. Despite their mild diuretic effect, the fluid in coffee, tea, and most other drinks still contributes to your daily total, and water-rich foods like fruit and vegetables add more. Plain water is simplest to track, but it is not the only source.
Can you drink too much water?: It is possible but uncommon. Drinking very large volumes in a short time can dilute blood sodium (hyponatraemia), which is dangerous. Spreading your intake through the day and drinking to thirst rather than forcing litres at once keeps you safe.

Related calculators

Disclaimer. This is general guidance, not medical advice. Individual water needs vary with climate, activity, body size, diet, and health conditions such as kidney or heart disease and pregnancy. Check with a healthcare professional if you have specific concerns.